1. Field of the Invention
The present invention relates to papermakers belts or fabrics, in general, and to a papermakers belt having smooth surfaces and an enlarged seam loop as defined by the inclusion of warp yarns of non-circular cross section, in particular.
2. Description of the Prior Art
In the dryer section of a papermaking machine, endless fabrics referred to in the industry as papermakers belts, dryer felts or dryer fabrics are employed to convey a paper web through the dryer section in order to remove moisture from the web. In the dryer section, the papermakers belt must carefully support and guide the web.
In use, a papermakers belt in the dryer section comes into contact with a number of heated cylinders. The paper web, being dried, is sandwiched between the dryer felt and the heated cylinders. The better the contact of the paper web to the heated cylinders, the better and more efficient the drying.
Within limits, increasing the tension of the dryer felt increases the contact between the paper web and heated cylinders. However, above an optimum tension, there is little improvement in contact between the dryer felt, paper web and heated cylinders. In fact, high fabric tensions have the marked disadvantage of likely causing distortion in the fabric, which leads to fabric narrowing and changes in permeability. It is important that the papermakers belt be of uniform construction throughout its structure and of proper tension when on the papermaking machine, so as to prevent undulations or cockling in the paper sheet being dried.
One type of prior art fabric commonly used in the dryer section of a papermaking machine is a duplex weave having two planes, each defined by a different plurality of cross-machine direction or weft yarns. A plurality of machine direction or warp yarns are interwoven with the weft yarns in accordance with a standard weave pattern to define knuckles or single floats on both the paper-contacting and non-paper-contacting surfaces of the fabric. When used in the dryer section of a papermaking machine, the warp ends of the duplex fabric are joined together by any conventional means such as through the formation of a pin seam or the use of various sewn on seams, such as, clipper, spiral, or multiloop seams.
Because the standard duplex weave has a knuckle structure on both the paper and non-paper side, ability of the fabric to hold the paper web in uniform intimate contact with the heated cylinders is limited. This is because the intimate contact of fabric to paper to cylinder occurs at the knuckle peaks. In addition, the valleys between the knuckle peaks permit the presence of air, which further reduces drying efficiency.
Should a pin seam be selected as the means for joining the ends of the fabric to form a continuous belt, the conventional duplex fabric produces a small seam loop which makes the hand sewing operation for joining the ends of the fabric together extremely tedious and time consuming, thus, increasing the costs of downtime on the paper machine.
In yet another common type of duplex dryer fabric there is provided a two-layered structure with separate weft yarns forming top and bottom layers. In this fabric, warp yarns define floats, which span at least two weft yarns, on the paper (or top) surface of the fabric and warp knuckles or single-floats on the non-paper (or bottom) surface of the fabric. Because of the structure of the smooth paper surface fabric, there are no free areas in which to insert stuffer picks. All of the areas defined by the warp yarns are filled with a cross-machine direction weft yarn. Therefore, the common smooth face duplex weaves do not permit changing permeability by use of a range of stuffer picks.
Thus, each time a different permeability characteristic is desired by a papermaker for a specific application, the fabric manufacturer must change the warp and weft yarns used to weave these styles of duplex fabrics. Such an undertaking by the manufacturer contributes to increasing the cost of the finished fabric. This is to be contrasted against the use of stuffer picks of various constructions which permit the manufacturer to leave the warp and weft yarns unchanged.
Further, should a pin seam be selected, the individual seam loops, formed by the warp yarns, have a tendency to move out of the plane of the fabric and thus cause peaks along the seam. These peaks (or loop knuckles) can be pressed into the paper sheet causing marking of the paper. At the same time, the proud loops are prone to be abraded by rolls in the paper machine run causing premature failure at the seam when the loops are worn through.
Yet another example of a way to control permeability in a dryer felt is the incorporation of warp yarns of rectangular cross section into a weave pattern that does not include provision for stuffer picks. In such a weave pattern, the warp yarns on the paper-receiving surface of the fabric typically float over a plurality of weft picks. The larger the float, i.e., the more picks the warp crosses before weaving back into the fabric, the less stable the fabric becomes. In this way, there is a trade off between permeability and fabric stability.
In the duplex fabrics discussed above, the non-paper side of the fabric comes into contact with numerous machine rolls between the heated cylinders and also on the return run. Unlike the paper side, there is no paper sheet sandwiched between the non-paper side of the fabric and these machine rolls, nor are the rolls driven. The non-paper side is therefore abraded by driving these free-rotating rolls (due to roll/fabric slippage). These rolls also tend to become rusted, and this is another source of abrasion. On the duplex fabrics, the abrasion takes place on the non-paper side knuckles and can thereby be fairly rapid.
There is thus a need for a papermakers belt which is capable of providing a smooth surface on the paper side so that it may find universal application for varying grades of paper, while at the same time having a smooth non-paper contacting surface to improve resistance to abrasion and guidability of the fabric. In addition, the fabric should incorporate the ability to use various types of stuffer picks so that the basic yarns used to form the fabric may remain unchanged. Finally, the seam loop formed in the fabric should remain in the plane of the fabric and thus eliminate undulations along the pin seam. The present invention is directed toward filling that need.